Scientists and clinicians face a number of challenges in their attempts to develop active agents into forms suited for delivery to a patient. Active agents that are proteins, for example, are often delivered via injection rather than orally. In this way, the protein is introduced into the systemic circulation without exposure to the proteolytic environment of the stomach. Injection of proteins, however, has several drawbacks. For example, many proteins have a relatively short half-life, thereby necessitating repeated injections, which are often inconvenient and painful. Moreover, some proteins may elicit one or more immune responses with the consequence that the patient's immune system attempts to destroy or otherwise neutralize the immunogenic protein. Of course, once the protein has been destroyed or otherwise neutralized, the protein cannot exert its intended pharmacodynamic activity. Thus, delivery of active agents such as proteins is often problematic even when these agents are administered by injection.
Some success has been achieved in addressing the problems of delivering active agents via injection. For example, conjugating an active agent to a water-soluble polymer has resulted in a polymer-active agent conjugate having reduced immunogenicity and antigenicity. In addition, the polymer-active agent conjugate often has an increased half-life compared to its unconjugated counterpart as a result of decreased clearance through the kidney and/or decreased enzymatic degradation in the systemic circulation. As a result of having a greater half-life, the polymer-active agent conjugate requires less frequent dosing, which in turn reduces the overall number of painful injections and inconvenient visits with a health care professional. Moreover, active agents that were only marginally soluble demonstrate a significant increase in water solubility when conjugated to a water-soluble polymer.
Due to its documented safety as well as its approval by the FDA for both topical and internal use, polyethylene glycol has been conjugated to active agents. When an active agent is conjugated to a polymer of polyethylene glycol or “PEG,” the conjugated active agent is conventionally referred to as “PEGylated.” The commercial success of PEGylated active agents such as PEGASYS® PEGylated interferon alpha-2a (Hoffmann-La Roche, Nutley, N.J.), PEG-INTRON® PEGylated interferon alpha-2b (Schering Corp., Kennilworth, N.J.), and NEULASTA™ PEG-filgrastim (Amgen Inc., Thousand Oaks, Calif.) demonstrates that administration of a conjugated form of an active agent can have significant advantages over the unconjugated counterpart. Small molecules such as distearoylphosphatidylethanolamine (Zalipsky (1993) Bioconjug. Chem. 4(4):296-299) and fluorouracil (Ouchi et al. (1992) Drug Des. Discov. 9(1):93-105) have also been PEGylated. Harris et al. have provided a review of the effects of PEGylation on pharmaceuticals. Harris et al. (2003) Nat. Rev. Drug Discov. 2(3):214-221.
Typically, the formation of a conjugate involves reaction between an active agent and a polymeric reagent. While small scale amounts of polymeric reagents are available from commercial sources such as Nektar Therapeutics, a concern arises when a commercial or production scale of the polymeric reagent is required. In particular, there is a concern that the particular polymeric reagent used in making the desired conjugate (or an intermediate useful in preparing the polymeric reagent used in making the desired conjugate) cannot be synthesized, largely free of potentially harmful impurities, in a timely, efficient, and economical manner.
For example, the conventional synthesis of polymers bearing an active ester—which can be used a polymeric reagent as well as an intermediate useful in preparing other polymeric reagents—requires an excess of a low molecular weight reagent [such as di(1-benzotriazolyl) carbonate], which must be removed. Although complicated, removal of the low molecular weight reagent is necessary so that the low molecular weight reagent does not react with other molecules, thereby introducing undesired side reactions and products that result in a relatively impure product and decreased yield.
In one approach for preparing polymers bearing an active ester, U.S. Pat. No. 6,624,246 describes the synthesis of methoxy poly(ethylene glycol) bearing a benzotriazole carbonate group (“mPEG-BTC”). As described therein, the process effectively involves mPEG-BTC formation followed by mPEG-BTC purification to purify the mPEG-BTC species and remove unreacted di(1-benzotriazolyl) carbonate and any other low molecular weight products (e.g., 1-benzotriazolyl alcohol). As described in U.S. Pat. No. 6,624,246, the purification of mPEG-BTC involves multiple precipitation steps. A schematic of the process is provided below.
                B) mPEG-BTC Purification                    a) distill off the organic solvent to form a residue of the three products            b) re-dissolve the three products in methylene chloride            c) add ethyl ether, cool and form a precipitate of                        

In the manufacture of mPEG-BTC wherein the poly(ethylene glycol) portion has a weight-average molecular weight of about 20,000 Daltons, an eight-fold excess of diBTC was used in order to achieve 100% conversion of all mPEG-OH to mPEG-BTC. Although the large amount of diBTC ensures optimal conversion to MPEG-OH to mPEG-BTC, a relatively large amount of diBTC remains unreacted and must be removed prior to carrying out any further synthetic steps. Otherwise, the remaining diBTC would react with any reactive group (e.g., alcohol group, amine group, and so forth) encountered and introduce undesired impurities and reduce the overall yield.
As described in U.S. Pat. No. 5,932,462, mPEG-BTC was reacted with lysine (bearing two amino groups and a single carboxylic acid group), thereby providing a “lysine branched” structure wherein an mPEG residue is attached at each of the amino groups and the carboxylic acid is available for further functionalizing. While it is conceivable to consume any excess diBTC with adding an excess of lysine, such an approach is flawed for at least two reasons. First, both the diBTC and mPEG-BTC will “compete” for the available lysine amino groups, thereby resulting in a mixture of the desired lysine branched structure and another species having only a single mPEG residue, a single BTC group, and a single carboxylic acid. Second, even if this approach was successful, it would not address situations where a non-lysine residue-containing product is desired.
One may avoid having excess diBTC during the lysine reaction only by destroying the excess diBTC from formation of mPEG-BTC during mPEG-BTC purification. Isopropyl alcohol (IPA, isopropanol) may be substituted for ethyl ether to precipitate the mPEG-BTC from a methylene chloride solution [i.e., e.g., step B(c) in the above schematic]. If this change is made, the IPA reacts with the excess diBTC to form isopropyl-BTC, which is soluble in the mixture of methylene chloride and IPA. Unfortunately, in such an operation at large scale, some isopropyl-BTC is trapped in the mPEG-BTC precipitate. Since the isopropyl-BTC would compete with mPEG-BTC in any reaction with lysine, the isopropyl-BTC must be removed before manufacturing can continue. To remove the trapped isopropyl-BTC, one or two additional “re-precipitation” steps must be carried out in order to get mPEG-BTC free of isopropyl-BTC. As each re-precipitation gives some loss of the mPEG-BTC product because only about 85-95% of the solid can be recovered, this approach is costly and requires additional time.
In another approach for preparing polymers bearing an active ester, U.S. Pat. No. 5,281,698 describes the synthesis of methoxy poly(ethylene glycol) bearing a succinimide carbonate group (“mPEG-SC”). As described therein, the process effectively involves mPEG-SC formation followed by mPEG-SC purification to purify the mPEG-SC species and remove unreacted disuccinimidyl carbonate and any other low molecular weight products (e.g., N-hydroxysuccinimide). As described in U.S. Pat. No. 5,281,698, the purification of mPEG-SC involves filtration and multiple precipitation steps. A schematic of the process is provided below.                A) mPEG-SC Formation        
                B) mPEG-SC Purification                    a) filter the reaction mixture            b) distill off the organic solvent to form a residue of the three products            c) re-dissolve the three products in methylene chloride            d) add ethyl ether, cool and form a precipitate of mPEG-SC            f) repeate precipitation (step c and d) two more times                        
In the manufacture of mPEG-SC wherein the poly(ethylene glycol) portion has a weight-average molecular weight of about 6100 Daltons, an twenty-fold excess of DSC was used in order to achieve 100% conversion of all mPEG-OH to mPEG-SC. Although the large amount of DSC ensures optimal conversion to MPEG-OH to mPEG-SC, a relatively large amount of DSC remains unreacted and must be removed prior to carrying out any further synthetic steps. Otherwise, the remaining DSC would react with any reactive group (e.g., alcohol group, amine group, and so forth) encountered and introduce undesired impurities and reduce the overall yield.
Thus, there remains a need for an efficient method to remove excess low molecular weight reagents, such as diBTC and/or its reactive degradants, from the same reaction mixture in which the low molecular weight reagent was added, thereby resulting in a “one-pot” reaction. This present invention addresses this and other needs in the art.